Impedance transformation network for improved driver circuit performance

ABSTRACT

This disclosure provides systems, methods and apparatus for reducing harmonic emissions. One aspect of the disclosure provides a transmitter apparatus. The transmitter apparatus includes a driver circuit characterized by an efficiency and a power output level. The driver circuit further includes a filter circuit electrically connected to the driver circuit and configured to modify the impedance of the transmit circuit to maintain the efficiency of the driver circuit at a level that is within 20% of a maximum efficiency of the driver circuit when the impedance is within the complex impedance range. The filter circuit is further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range. The filter circuit is further configured to maintain a substantially linear relationship between the power output level and the resistive variations within the impedance range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application No.61/769,152, entitled “IMPEDANCE TRANSFORMATION NETWORK FOR IMPROVEDDRIVER CIRCUIT PERFORMANCE,” filed Feb. 25, 2013, which is herebyexpressly incorporated by reference herein. This application is relatedto U.S. patent application Ser. No. 13/424,834 entitled “FILTER FORIMPROVED DRIVER CIRCUIT EFFICIENCY AND METHOD OF OPERATION” filed Mar.20, 2012; and U.S. patent application Ser. No. 13/625,813 entitled“FILTER FOR IMPROVED DRIVER CIRCUIT EFFICIENCY AND METHOD OF OPERATION”filed Sep. 24, 2012; the disclosures of both of which are herebyincorporated by reference in their entirety.

FIELD

The present invention relates generally to wireless power. Morespecifically, the disclosure is directed to improving the efficiency andpower output of transmit circuit driving a load varying over a wideresistive and reactive range.

BACKGROUND

An increasing number and variety of electronic devices are powered viarechargeable batteries. Such devices include mobile phones, portablemusic players, laptop computers, tablet computers, computer peripheraldevices, communication devices (e.g., Bluetooth devices), digitalcameras, hearing aids, and the like. While battery technology hasimproved, battery-powered electronic devices increasingly require andconsume greater amounts of power. As such, these devices constantlyrequire recharging. Rechargeable devices are often charged via wiredconnections that require cables or other similar connectors that arephysically connected to a power supply. Cables and similar connectorsmay sometimes be inconvenient or cumbersome and have other drawbacks.Wireless charging systems that are capable of transferring power in freespace to be used to charge rechargeable electronic devices may overcomesome of the deficiencies of wired charging solutions. As such, wirelesscharging systems and methods that efficiently and safely transfer powerfor charging rechargeable electronic devices are desirable.

SUMMARY

Various implementations of systems, methods and devices within the scopeof the appended claims each have several aspects, no single one of whichis solely responsible for the desirable attributes described herein.Without limiting the scope of the appended claims, some prominentfeatures are described herein.

Details of one or more implementations of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

One aspect of the subject matter described in the disclosure provides atransmitter apparatus. The transmitter apparatus includes a drivercircuit characterized by an efficiency and a power output level. Thedriver circuit is electrically connected to a transmit circuit having animpedance. The impedance of the transmit circuit is within a compleximpedance range including resistive and reactive variations. The compleximpedance range is defined by a minimum real impedance value, a maximumreal impedance, a minimum imaginary impedance value, and a maximumimaginary impedance value. A ratio between the minimum and maximum realimpedance value is at least two to one. A magnitude of the differencebetween the maximum and minimum imaginary impedance values being atleast twice a magnitude of the difference between the minimum andmaximum real impedance values. The transmitter apparatus furtherincludes a filter circuit electrically connected to the driver circuitand configured to modify the impedance of the transmit circuit tomaintain the efficiency of the driver circuit at a level that is within20% of a maximum efficiency of the driver circuit when the impedance iswithin the complex impedance range. The filter circuit is furtherconfigured to maintain a substantially constant power output levelirrespective of the reactive variations within the complex impedancerange. The filter circuit is further configured to maintain asubstantially linear relationship between the power output level and theresistive variations within the complex impedance range.

Another aspect of the subject matter described in the disclosureprovides a transmitter apparatus. The transmitter apparatus includes adriver circuit including a switching amplifier circuit comprising aswitch, a switch shunt capacitor, and a series inductor electricallyconnected to the output of the driver circuit. The transmitter apparatusfurther includes a transmit circuit including a coil having aninductance electrically connected in series to a capacitor to form aresonant circuit. The transmitter apparatus further includes a filtercircuit electrically connected between the driver circuit and thetransmit circuit, the filter circuit comprising solely of a single shuntcapacitor network.

Yet another aspect of the subject matter described in the disclosureprovides a method of selecting component values of one or more reactivecomponents of a filter circuit for a wireless power transmitter device.The filter circuit is electrically connected between a driver circuitand a transmit circuit. The method includes determining a first set ofcomplex impedance values for which efficiency of the driver circuit isabove a threshold. The first set of complex impedance valuessubstantially map to complex impedance values along a half circle path.The method further includes determining a second set of compleximpedance values for which power output of the driver circuit issubstantially constant. The second set of complex impedance valuessubstantially map to values along a full circle path that is orthogonalto the half circle and which crosses the half circle at a maximum. Themethod further includes selecting the component values to provide animpedance transformation that modifies a variable complex impedance ofthe transmit circuit to complex impedance values derived from the firstand second sent of complex impedance values.

Another aspect of the subject matter described in the disclosureprovides a transmitter apparatus. The transmitter apparatus includes adriver circuit characterized by an efficiency and a power output level.The driver circuit is electrically connected to a transmit circuithaving an impedance. The impedance of the transmit circuit is within acomplex impedance range including resistive and reactive variations. Thetransmitter apparatus further includes a filter circuit electricallyconnected to the driver circuit and configured to modify the impedanceof the transmit circuit. The filter circuit has one or more reactivecomponents with values selected derived from a first value and a secondvalue. The first value, Rd, corresponds to a radius of a half circle.The half circle is defined by a set of complex impedance values alongthe perimeter of the half circle that correspond to values for whichefficiency of the driver circuit is at least within 20% of the maximumefficiency of the driver circuit. The second value R0, corresponds to areal impedance value at the load of the filter circuit that results in adesired transformed impedance being equal to Rd at an input of thefilter circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an exemplary wireless powertransfer system, in accordance with exemplary embodiments of theinvention.

FIG. 2 is a functional block diagram of exemplary components that may beused in the wireless power transfer system of FIG. 1, in accordance withvarious exemplary embodiments of the invention.

FIG. 3 is a schematic diagram of a portion of transmit circuitry orreceive circuitry of FIG. 2 including a transmit or receive coil, inaccordance with exemplary embodiments of the invention.

FIG. 4 is a functional block diagram of a transmitter that may be usedin the wireless power transfer system of FIG. 1, in accordance withexemplary embodiments of the invention.

FIG. 5 is a functional block diagram of a receiver that may be used inthe wireless power transfer system of FIG. 1, in accordance withexemplary embodiments of the invention.

FIG. 6 is a functional block diagram of an exemplary wireless powertransfer system as in FIG. 2, where a transmitter may wirelessly providepower to multiple receivers, in accordance with various exemplaryembodiments of the invention.

FIG. 7 is a schematic diagram of a driver circuit that may be used inthe transmitter of FIG. 6, in accordance with exemplary embodiments ofthe invention.

FIG. 8A is a diagram showing an exemplary range of complex impedancesthat may be presented to the driver circuit during operation of awireless power transmitter.

FIG. 8B is a plot showing efficiency and output power of the drivercircuit of FIG. 7 as a function of the real impedance of a load of adriver circuit.

FIG. 9 is a contour plot showing the efficiency of a driver circuit asin FIG. 7 as a function of the real and imaginary components of the loadimpedance presented to the driver circuit.

FIG. 10A is a contour plot showing the power output of a driver circuitas in FIG. 7 and a maximum efficiency contour as a function of real andimaginary components of the load impedance presented to the drivercircuit.

FIG. 10B is another plot showing a selected power output contour and amaximum efficiency contour for a driver circuit as in FIG. 7 as afunction of real and imaginary components of the load impedancepresented to the driver circuit.

FIGS. 11A, 11B, 12A, and 12B show corresponding measured results ascompared to FIG. 10B showing power output and efficiency of a drivercircuit as in FIG. 7 as a function of real and imaginary components ofthe load impedance presented to the driver circuit.

FIG. 13 is a schematic diagram of a driver circuit as in FIG. 7including a filter circuit, in accordance with exemplary embodiments ofthe invention.

FIG. 14 is a schematic diagram of the circuit of FIG. 13 in accordancewith an embodiment.

FIG. 15 is a plot showing the impedance transformed by the filtercircuit versus the impedance presented to the transmit circuit as mappedto a high efficiency contour.

FIG. 16 is a plot showing the impedance transformed by the filtercircuit over a large real range presented to the transmit circuit.

FIG. 17 is a contour plot showing the maximum efficiency of a drivercircuit as a function of the real and imaginary components of the loadimpedance presented to the driver circuit.

FIG. 18 is a plot showing measured impedance transformed by a filtercircuit overlaid on calculated values for which power is substantiallyconstant and efficiency is maximum.

FIG. 19 is a plot showing the impedance transformed by a filter circuitwhen presented with a complex impedance range.

FIG. 20 shows the measured paths of a result of a filter circuitoverlaid on the data of FIG. 12B.

FIG. 21 shows the measured paths of a result of a filter circuitoverlaid on the data of FIG. 11B.

FIG. 22 is a flowchart of an exemplary method for designing a highlyefficient transmit circuit.

FIG. 23 is another schematic diagram of portion of transmit circuitry,in accordance with an exemplary embodiment of the invention.

FIG. 24 shows measured results for power output from a driver circuit asin FIG. 13 as a function of real and imaginary components of the loadimpedance presented to the driver circuit.

The various features illustrated in the drawings may not be drawn toscale. Accordingly, the dimensions of the various features may bearbitrarily expanded or reduced for clarity. In addition, some of thedrawings may not depict all of the components of a given system, methodor device. Finally, like reference numerals may be used to denote likefeatures throughout the specification and figures.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of theinvention and is not intended to represent the only embodiments in whichthe invention may be practiced. The term “exemplary” used throughoutthis description means “serving as an example, instance, orillustration,” and should not necessarily be construed as preferred oradvantageous over other exemplary embodiments. The detailed descriptionincludes specific details for the purpose of providing a thoroughunderstanding of the exemplary embodiments of the invention. Theexemplary embodiments of the invention may be practiced without thesespecific details. In some instances, well-known structures and devicesare shown in block diagram form in order to avoid obscuring the noveltyof the exemplary embodiments presented herein.

Wirelessly transferring power may refer to transferring any form ofenergy associated with electric fields, magnetic fields, electromagneticfields, or otherwise from a transmitter to a receiver without the use ofphysical electrical conductors (e.g., power may be transferred throughfree space). The power output into a wireless field (e.g., a magneticfield) may be received, captured by, or coupled by a “receiving coil” toachieve power transfer.

FIG. 1 is a functional block diagram of an exemplary wireless powertransfer system 100, in accordance with exemplary embodiments of theinvention. Input power 102 may be provided to a transmitter 104 from apower source (not shown) for generating a field 106 for providing energytransfer. A receiver 108 may couple to the field 106 and generate outputpower 110 for storing or consumption by a device (not shown) coupled tothe output power 110. Both the transmitter 104 and the receiver 108 areseparated by a distance 112. In one exemplary embodiment, transmitter104 and receiver 108 are configured according to a mutual resonantrelationship. When the resonant frequency of receiver 108 and theresonant frequency of transmitter 104 are substantially the same or veryclose, transmission losses between the transmitter 104 and the receiver108 are minimal. As such, wireless power transfer may be provided overlarger distance in contrast to purely inductive solutions that mayrequire large coils that require coils to be very close (e.g., mms).Resonant inductive coupling techniques may thus allow for improvedefficiency and power transfer over various distances and with a varietyof inductive coil configurations.

The receiver 108 may receive power when the receiver 108 is located inan energy field 106 produced by the transmitter 104. The field 106corresponds to a region where energy output by the transmitter 104 maybe captured by a receiver 106. In some cases, the field 106 maycorrespond to the “near-field” of the transmitter 104 as will be furtherdescribed below.

The transmitter 104 may include a transmit coil 114 for outputting anenergy transmission. The receiver 108 further includes a receive coil118 for receiving or capturing energy from the energy transmission. Thenear-field may correspond to a region in which there are strong reactivefields resulting from the currents and charges in the transmit coil 114that minimally radiate power away from the transmit coil 114. In somecases the near-field may correspond to a region that is within about onewavelength (or a fraction thereof) of the transmit coil 114. Thetransmit and receive coils 114 and 118 are sized according toapplications and devices to be associated therewith. As described above,efficient energy transfer may occur by coupling a large portion of theenergy in a field 106 of the transmit coil 114 to a receive coil 118rather than propagating most of the energy in an electromagnetic wave tothe far field. When positioned within the field 106, a “coupling mode”may be developed between the transmit coil 114 and the receive coil 118.The area around the transmit and receive coils 114 and 118 where thiscoupling may occur is referred to herein as a coupling-mode region.

FIG. 2 is a functional block diagram of exemplary components that may beused in the wireless power transfer system 100 of FIG. 1, in accordancewith various exemplary embodiments of the invention. The transmitter 204may include transmit circuitry 206 that may include an oscillator 222, adriver circuit 224, and a filter and impedance transforming circuit 226.The oscillator 222 may be configured to generate a signal at a desiredfrequency, such as 468.75 KHz, 6.78 MHz or 13.56 MHz, that may beadjusted in response to a frequency control signal 223. The oscillatorsignal may be provided to a driver circuit 224 configured to drive thetransmit coil 214 at, for example, a resonant frequency of the transmitcoil 214. The driver circuit 224 may be a switching amplifier configuredto receive a square wave from the oscillator 222 and output a sine wave.For example, the driver circuit 224 may be a class E amplifier. A filterand impedance transforming circuit 226 may be also included to filterout harmonics or other unwanted frequencies and match the impedance ofthe transmitter 204 to the transmit coil 214. The filter and impedancetransforming circuit 226 may be configured to perform a variety ofimpedance adjustments other than just matching the impedance of thetransmitter 204 to the transmit coil 214.

The receiver 208 may include receive circuitry 210 that may include amatching circuit 232 (or any other type of impedance adjustment circuit)and a rectifier and switching circuit 234 to generate a DC power outputfrom an AC power input to charge a battery 236 as shown in FIG. 2 or topower a device (not shown) coupled to the receiver 108. The matchingcircuit 232 may be included to match the impedance of the receivecircuitry 210 to the receive coil 218. The receiver 208 and transmitter204 may additionally communicate on a separate communication channel 219(e.g., Bluetooth, zigbee, cellular, etc.). The receiver 208 andtransmitter 204 may alternatively communicate via in-band signalingusing characteristics of the wireless field 206.

As described more fully below, receiver 208, that may initially have aselectively disablable associated load (e.g., battery 236), may beconfigured to determine whether an amount of power transmitted bytransmitter 204 and receiver by receiver 208 is appropriate for charginga battery 236. Further, receiver 208 may be configured to enable a load(e.g., battery 236) upon determining that the amount of power isappropriate. In some embodiments, a receiver 208 may be configured todirectly utilize power received from a wireless power transfer fieldwithout charging of a battery 236. For example, a communication device,such as a near-field communication (NFC) or radio-frequencyidentification device (RFID) may be configured to receive power from awireless power transfer field and communicate by interacting with thewireless power transfer field and/or utilize the received power tocommunicate with a transmitter 204 or other devices

FIG. 3 is a schematic diagram of a portion of transmit circuitry orreceive circuitry of FIG. 2 including a transmit or receive coil 352, inaccordance with exemplary embodiments of the invention. As illustratedin FIG. 3, transmit circuitry 350 used in exemplary embodiments mayinclude a coil 352. The coil may also be referred to or be configured asa “loop” antenna 352. The coil 352 may also be referred to herein orconfigured as a “magnetic” antenna or an induction coil. The term “coil”is intended to refer to a component that may wirelessly output orreceive energy for coupling to another “coil.” The coil may also bereferred to as an “antenna” of a type that is configured to wirelesslyoutput or receive power. The coil may also be referred to as a wirelesspower transfer component of a type that is configured to wirelesslytransmit or receive power. The coil 352 may be configured to include anair core or a physical core such as a ferrite core (not shown).

As stated, efficient transfer of energy between the transmitter 104 andreceiver 108 may occur during matched or nearly matched resonancebetween the transmitter 104 and the receiver 108. However, even whenresonance between the transmitter 104 and receiver 108 are not matched,energy may be transferred, although the efficiency may be affected.Transfer of energy occurs by coupling energy from the field 106 of thetransmitting coil to the receiving coil residing in the neighborhoodwhere this field 106 is established rather than propagating the energyfrom the transmitting coil into free space.

The resonant frequency of the loop or magnetic coils is based on theinductance and capacitance. Inductance may be simply the inductancecreated by the coil 352, whereas, capacitance may be added to the coil'sinductance to create a resonant structure at a desired resonantfrequency. As a non-limiting example, capacitor 354 and capacitor 356may be added to the transmit circuitry 350 to create a resonant circuitthat selects a signal 358 at a resonant frequency. Accordingly, forlarger diameter coils, the size of capacitance needed to sustainresonance may decrease as the diameter or inductance of the loopincreases. Furthermore, as the diameter of the coil increases, theefficient energy transfer area of the near-field may increase. Otherresonant circuits formed using other components are also possible. Asanother non-limiting example, a capacitor may be placed in parallelbetween the two terminals of the coil 350. For transmit coils, a signal358 with a frequency that substantially corresponds to the resonantfrequency of the coil 352 may be an input to the coil 352.

In one embodiment, the transmitter 104 (FIG. 1) may be configured tooutput a time varying magnetic field with a frequency corresponding tothe resonant frequency of the transmit coil 114. When the receiver iswithin the field 106, the time varying magnetic field may induce acurrent in the receive coil 118. As described above, if the receive coil118 is configured to be resonant at the frequency of the transmit coil118, energy may be efficiently transferred. The AC signal induced in thereceive coil 118 may be rectified as described above to produce a DCsignal that may be provided to charge or to power a load.

FIG. 4 is a functional block diagram of a transmitter 404 that may beused in the wireless power transfer system of FIG. 1, in accordance withexemplary embodiments of the invention. The transmitter 404 may includetransmit circuitry 406 and a transmit coil 414. The transmit coil 414may be the coil 352 as shown in FIG. 3. Transmit circuitry 406 mayprovide RF power to the transmit coil 414 by providing an oscillatingsignal resulting in generation of energy (e.g., magnetic flux) about thetransmit coil 414. Transmitter 404 may operate at any suitablefrequency. By way of example, transmitter 404 may operate at the 6.78MHz ISM band.

Transmit circuitry 406 may include a fixed impedance matching circuit406 for matching the impedance of the transmit circuitry 406 (e.g., 50ohms) to the transmit coil 414 and a low pass filter (LPF) 408configured to reduce harmonic emissions to levels to preventself-jamming of devices coupled to receivers 108 (FIG. 1). Otherexemplary embodiments may include different filter topologies, includingbut not limited to, notch filters that attenuate specific frequencieswhile passing others and may include an adaptive impedance match, thatmay be varied based on measurable transmit metrics, such as output powerto the coil 414 or DC current drawn by the driver circuit 424. Transmitcircuitry 406 further includes a driver circuit 424 configured to drivean RF signal as determined by an oscillator 422. The transmit circuitry406 may be comprised of discrete devices or circuits, or alternately,may be comprised of an integrated assembly. An exemplary RF power outputfrom transmit coil 414 may be on the order of 2.5 Watts.

Transmit circuitry 406 may further include a controller 410 forselectively enabling the oscillator 422 during transmit phases (or dutycycles), for adjusting the frequency or phase of the oscillator 422, andfor adjusting the output power level for implementing a communicationprotocol for interacting with neighboring devices through their attachedreceivers. It is noted that the controller 410 may also be referred toherein as processor 410. Adjustment of oscillator phase and relatedcircuitry in the transmission path may allow for reduction of out ofband emissions.

The transmit circuitry 406 may further include a load sensing circuit416 for detecting the presence or absence of active receivers in thevicinity of the near-field generated by transmit coil 414. By way ofexample, a load sensing circuit 416 monitors the current flowing to thedriver circuit 424, that may be affected by the presence or absence ofactive receivers in the vicinity of the field generated by transmit coil414 as will be further described below. Detection of changes to theloading on the driver circuit 424 are monitored by controller 410 foruse in determining whether to enable the oscillator 422 for transmittingenergy and to communicate with an active receiver. As described morefully below, a current measured at the driver circuit 424 may be used todetermine whether an invalid device is positioned within a wirelesspower transfer region of the transmitter 404.

The transmit coil 414 may be implemented with a Litz wire or as anantenna strip with the thickness, width and metal type selected to keepresistive losses low. In a one implementation, the transmit coil 414 maygenerally be configured for association with a larger structure such asa table, mat, lamp or other less portable configuration. Accordingly,the transmit coil 414 generally may not need “turns” in order to be of apractical dimension. An exemplary implementation of a transmit coil 414may be “electrically small” (i.e., fraction of the wavelength) and tunedto resonate at lower usable frequencies by using capacitors to definethe resonant frequency.

The transmitter 404 may gather and track information about thewhereabouts and status of receiver devices that may be associated withthe transmitter 404. Thus, the transmitter circuitry 404 may include apresence detector 480, an enclosed detector 460, or a combinationthereof, connected to the controller 410 (also referred to as aprocessor herein). The controller 410 may adjust an amount of powerdelivered by the driver circuit 424 in response to presence signals fromthe presence detector 480 and the enclosed detector 460. The transmitter404 may receive power through a number of power sources, such as, forexample, an AC-DC converter (not shown) to convert conventional AC powerpresent in a building, a DC-DC converter (not shown) to convert aconventional DC power source to a voltage suitable for the transmitter404, or directly from a conventional DC power source (not shown).

As a non-limiting example, the presence detector 480 may be a motiondetector utilized to sense the initial presence of a device to becharged that is inserted into the coverage area of the transmitter 404.After detection, the transmitter 404 may be turned on and the RF powerreceived by the device may be used to toggle a switch on the Rx devicein a pre-determined manner, which in turn results in changes to thedriving point impedance of the transmitter 404.

As another non-limiting example, the presence detector 480 may be adetector capable of detecting a human, for example, by infrareddetection, motion detection, or other suitable means. In some exemplaryembodiments, there may be regulations limiting the amount of power thata transmit coil 414 may transmit at a specific frequency. In some cases,these regulations are meant to protect humans from electromagneticradiation. However, there may be environments where a transmit coil 414is placed in areas not occupied by humans, or occupied infrequently byhumans, such as, for example, garages, factory floors, shops, and thelike. If these environments are free from humans, it may be permissibleto increase the power output of the transmit coil 414 above the normalpower restrictions regulations. In other words, the controller 410 mayadjust the power output of the transmit coil 414 to a regulatory levelor lower in response to human presence and adjust the power output ofthe transmit coil 414 to a level above the regulatory level when a humanis outside a regulatory distance from the electromagnetic field of thetransmit coil 414.

As a non-limiting example, the enclosed detector 460 (may also bereferred to herein as an enclosed compartment detector or an enclosedspace detector) may be a device such as a sense switch for determiningwhen an enclosure is in a closed or open state. When a transmitter is inan enclosure that is in an enclosed state, a power level of thetransmitter may be increased.

In exemplary embodiments, a method by which the transmitter 404 does notremain on indefinitely may be used. In this case, the transmitter 404may be programmed to shut off after a user-determined amount of time.This feature prevents the transmitter 404, notably the driver circuit424, from running long after the wireless devices in its perimeter arefully charged. This event may be due to the failure of the circuit todetect the signal sent from either the repeater or the receive coil thata device is fully charged. To prevent the transmitter 404 fromautomatically shutting down if another device is placed in itsperimeter, the transmitter 404 automatic shut off feature may beactivated only after a set period of lack of motion detected in itsperimeter. The user may be able to determine the inactivity timeinterval, and change it as desired. As a non-limiting example, the timeinterval may be longer than that needed to fully charge a specific typeof wireless device under the assumption of the device being initiallyfully discharged.

FIG. 5 is a functional block diagram of a receiver 508 that may be usedin the wireless power transfer system of FIG. 1, in accordance withexemplary embodiments of the invention. The receiver 508 includesreceive circuitry 510 that may include a receive coil 518. Receiver 508further couples to device 550 for providing received power thereto. Itshould be noted that receiver 508 is illustrated as being external todevice 550 but may be integrated into device 550. Energy may bepropagated wirelessly to receive coil 518 and then coupled through therest of the receive circuitry 510 to device 550. By way of example, thecharging device may include devices such as mobile phones, portablemusic players, laptop computers, tablet computers, computer peripheraldevices, communication devices (e.g., Bluetooth devices), digitalcameras, hearing aids (and other medical devices), and the like.

Receive coil 518 may be tuned to resonate at the same frequency, orwithin a specified range of frequencies, as transmit coil 414 (FIG. 4).Receive coil 518 may be similarly dimensioned with transmit coil 414 ormay be differently sized based upon the dimensions of the associateddevice 550. By way of example, device 550 may be a portable electronicdevice having diametric or length dimension smaller that the diameter oflength of transmit coil 414. In such an example, receive coil 518 may beimplemented as a multi-turn coil in order to reduce the capacitancevalue of a tuning capacitor (not shown) and increase the receive coil'simpedance. By way of example, receive coil 518 may be placed around thesubstantial circumference of device 550 in order to maximize the coildiameter and reduce the number of loop turns (i.e., windings) of thereceive coil 518 and the inter-winding capacitance.

Receive circuitry 510 may provide an impedance match to the receive coil518. Receive circuitry 510 includes power conversion circuitry 506 forconverting a received RF energy source into charging power for use bythe device 550. Power conversion circuitry 506 includes an RF-to-DCconverter 508 and may also in include a DC-to-DC converter 510. RF-to-DCconverter 508 rectifies the RF energy signal received at receive coil518 into a non-alternating power with an output voltage represented byV_(rect). The DC-to-DC converter 510 (or other power regulator) convertsthe rectified RF energy signal into an energy potential (e.g., voltage)that is compatible with device 550 with an output voltage and outputcurrent represented by V_(out) and I_(out). Various RF-to-DC convertersare contemplated, including partial and full rectifiers, regulators,bridges, doublers, as well as linear and switching converters.

Receive circuitry 510 may further include switching circuitry 512 forconnecting receive coil 518 to the power conversion circuitry 506 oralternatively for disconnecting the power conversion circuitry 506.Disconnecting receive coil 518 from power conversion circuitry 506 notonly suspends charging of device 550, but also changes the “load” as“seen” by the transmitter 404 (FIG. 2).

As disclosed above, transmitter 404 includes load sensing circuit 416that may detect fluctuations in the bias current provided to transmitterpower driver circuit 410. Accordingly, transmitter 404 has a mechanismfor determining when receivers are present in the transmitter'snear-field.

When multiple receivers 508 are present in a transmitter's near-field,it may be desirable to time-multiplex the loading and unloading of oneor more receivers to enable other receivers to more efficiently coupleto the transmitter. A receiver 508 may also be cloaked in order toeliminate coupling to other nearby receivers or to reduce loading onnearby transmitters. This “unloading” of a receiver is also known hereinas a “cloaking.” Furthermore, this switching between unloading andloading controlled by receiver 508 and detected by transmitter 404 mayprovide a communication mechanism from receiver 508 to transmitter 404as is explained more fully below. Additionally, a protocol may beassociated with the switching that enables the sending of a message fromreceiver 508 to transmitter 404. By way of example, a switching speedmay be on the order of 100 μsec.

In an exemplary embodiment, communication between the transmitter 404and the receiver 508 refers to a device sensing and charging controlmechanism, rather than conventional two-way communication (i.e., in bandsignaling using the coupling field). In other words, the transmitter 404may use on/off keying of the transmitted signal to adjust whether energyis available in the near-field. The receiver may interpret these changesin energy as a message from the transmitter 404. From the receiver side,the receiver 508 may use tuning and de-tuning of the receive coil 518 toadjust how much power is being accepted from the field. In some cases,the tuning and de-tuning may be accomplished via the switching circuitry512. The transmitter 404 may detect this difference in power used fromthe field and interpret these changes as a message from the receiver508. It is noted that other forms of modulation of the transmit powerand the load behavior may be utilized.

Receive circuitry 510 may further include signaling detector and beaconcircuitry 514 used to identify received energy fluctuations, that maycorrespond to informational signaling from the transmitter to thereceiver. Furthermore, signaling and beacon circuitry 514 may also beused to detect the transmission of a reduced RF signal energy (i.e., abeacon signal) and to rectify the reduced RF signal energy into anominal power for awakening either un-powered or power-depleted circuitswithin receive circuitry 510 in order to configure receive circuitry 510for wireless charging.

Receive circuitry 510 further includes processor 516 for coordinatingthe processes of receiver 508 described herein including the control ofswitching circuitry 512 described herein. Cloaking of receiver 508 mayalso occur upon the occurrence of other events including detection of anexternal wired charging source (e.g., wall/USB power) providing chargingpower to device 550. Processor 516, in addition to controlling thecloaking of the receiver, may also monitor beacon circuitry 514 todetermine a beacon state and extract messages sent from the transmitter404. Processor 516 may also adjust the DC-to-DC converter 510 forimproved performance.

FIG. 6 is a functional block diagram of an exemplary wireless powertransfer system 600 as in FIG. 2, where a transmitter 604 may wirelesslyprovide power to multiple receivers 608 a, 608 b, and 608 c, inaccordance with various exemplary embodiments of the invention. As shownin FIG. 6, a transmitter 604 may transmit power via a transmit coil 614via a field 606. Receiver devices 608 a, 608 b, and 608 c may receivewireless power by coupling a portion of energy from the field 606 usingreceive coils 618 a, 618 b, and 618 c to charge or power respectiveloads 636 a, 636 b, and 636 c. Furthermore, the transmitter 604 mayestablish communication links 619 a, 619 b, and 616 c with receivers 618a, 618 b, and 618 c respectively. While three receivers 608 a, 608 b,and 608 c are shown, additional receivers (not shown) may receive powerfrom the transmitter 604.

In a wireless power transfer system 600 the receivers 608 a, 608 b, or608 c may correspond to the load the transmitter drives whiletransferring power. As such, the load driven by the transmitter 604 maybe a function of each receiver 608 a, 608 b, or 608 c that is wirelesslyreceiving power from the field 606. When receivers 608 a, 608 b, or 608c enter the field 606, leave the field, or disable or enable theircapability to receive power from the field 606, the complex loadpresented to the transmitter 604 is altered accordingly. Both resistiveand reactive variations of the load are altered. The behavior of thetransmitter 604 may be a function of characteristics of the variablecomplex load. For example, the efficiency at which the transmitter 604may provide power to a receiver 608 a, 608 b, or 608 c may vary as thecomplex load of the transmitter 604 varies. Furthermore, the amount ofpower that the transmitter 604 outputs may also vary as the complex loadvaries. Each of the receivers 608 a, 608 b, and 608 c may form a portionof the load of the transmitter 404 when each receiver 608 a, 608 b, and608 c is receiving power via the field 606. The total impedance of theload seen by the transmit coil 614 may be a sum of impedances resultingfrom each receiver 608 a, 608 b, and 608 c as the impedances theypresent to the transmit circuit 614 may combine in series.

In one aspect, exemplary embodiments are directed to a transmitter 604that is suitable for efficiently charging a dynamic number of receivers608 a, 608 b, and 608 c. To efficiently allow for two receivers 608 aand 608 b to receive more power than when one receiver 608 a ispositioned to receive power, the transmitter 604 may be preferablydesigned such that the load (characterized by its complex impedance) atwhich the maximum power may be delivered is lower than the load at whichthe maximum transmitter efficiency may be provided. Furthermore, thetransmitter 604 may be preferably designed to provide power at highefficiency over a resistive and reactive range of complex load values asa variable number of receivers 608 a, 608 b, and 608 c will result in arange of different loads being presented to the transmitter 604.Otherwise, significant power losses may arise. Moreover, the transmitter604 may be preferably designed such that the load at which maximum poweris provided is greater than a total load presented by multiple receivers608 a, 608 b, and 608 c. In this case, the transmitter 604 may havesufficient power to supply multiple devices simultaneously.

The transmit circuit may be driven by a driver circuit. FIG. 7 is aschematic diagram of a driver circuit 724 that may be used in thetransmitter 604 of FIG. 6, in accordance with exemplary embodiments ofthe invention. As stated, the power output and efficiency of the drivercircuit (e.g., a driver circuit 424) varies as a function of the loadpresented to the driver circuit 724. In some embodiments, the drivercircuit 724 may be a switching amplifier. The driver circuit 724 may beconfigured to receive a square wave and output a sine wave to beprovided to the transmit circuit 750. The driver circuit 724 is shown asan ideal (i.e., no internal resistive losses) class E amplifier. Thedriver circuit 724 includes a switched shunt capacitor 710 and a seriesinductance 708. V_(D) is a DC source voltage applied to the drivercircuit 724 that controls the maximum power that may be delivered into aseries tuned load. The driver circuit 724 is driven by an oscillatinginput signal 702 to a switch 704.

While the driver circuit 724 is shown as a class E amplifier,embodiments in accordance with the invention may use other types ofdriving circuits as may be known by those skilled in the art. A drivercircuit 724 may be used to efficiently drive a load. The load may be atransmit circuit 750 configured to wirelessly transmit power. Thetransmit circuit 750 may include a series inductor 714 and capacitor 716to form a resonant circuit as described above with reference to FIG. 3.While the load is shown as a transmit circuit 750, embodiments inaccordance with the invention may be applicable to other loads. Asdescribed above with reference to FIG. 6, the load presented to thetransmit circuit 750 may be variable due to the number of wireless powerreceivers 608 a, 608 b, and 608 c and may be represented by a variableresistor 712 indicative of resistive variation of the load and avariable inductor 712 indicative of reactive variation of the load. Thedriver circuit 724 may be driven by an input signal 702, such as from anoscillator 222 (FIG. 2). As the load presented to the transmit circuit750 varies, for example, due to a dynamic number of wireless powerreceivers 638 a, 638 b, and 638 c as described above, a load presentedto the driver circuit 724 may also vary according to a wide resistanceand reactance range. For example, when an additional receiver 638 a ispositioned to receive power from the transmit circuit 750, the receiver638 a picking up power increases the resistance presented to thetransmit circuit 750 and therefore to the driver circuit 724. Inaddition, adding a receiver 638 b that includes certain material (e.g.,metal) may result in a large reactance swing presented to the transmitcircuit 750 and therefore to the driver circuit 724. For example certainlarge receivers (e.g., a tablet) may present a negative reactance swingon the order of an excess of −j100. Power provided by the driver circuit724 may not be flat across a load reactance range.

The load presented to the driver circuit 724 may be described by theimpedance presented to the transmit circuit 750 including both resistiveand reactive components and defined as Zin(TX)=Rin(TX)+jXin(TX). Thevalue of Zin(TX) depends on various factors such as the transmit coiland receive coil structures, the type and number of devices to becharged, the power demanded by each receiver, and the like. The range ofthe load may be defined by four corner impedances:

R _(IN) _(—) _(TX) _(—) _(MIN) ≦Re{Z _(IN) _(—) _(TX) }≦R _(IN) _(—)_(TX) _(—) _(MAX)

X _(IN) _(—) _(TX) _(—) _(MIN) ≦Im{Z _(IN) _(—) _(TX) }≦X _(IN) _(—)_(TX) _(—) _(MAX)

FIG. 8A is a diagram showing an exemplary range of impedances that maybe presented to the transmit circuit 750 during operation. FIG. 8A showsthe corner impedances as described above. According to the type andnumber of devices to charge, the possible values for the cornerimpedances may vary widely. For example, for purposes of illustration,R_(IN) _(—) _(TX) _(—) _(MIN) may be defined as 0Ω while R_(IN) _(—)_(TX) _(—) _(MAX) may be 75Ω. In addition, for purposes of illustration,X_(IN) _(—) _(TX) _(—) _(MIN) may be defined as −50 jΩ while X_(IN) _(—)_(TX) _(—) _(MAX) may be +50 jΩ. In accordance with another embodiment,R_(IN) _(—) _(TX) _(—) _(MIN) 0Ω, R_(IN) _(—) _(TX) _(—) _(MAX) issubstantially 200Ω, X_(IN) _(—) _(TX) _(—) _(MIN) is substantially −200jΩ, and X_(INT) _(—) _(TX) _(—) _(MAX) is substantially +200 jΩ. Theprinciples described herein may apply to these and other compleximpedance ranges. According to another exemplary embodiment, in anoperating mode the real load impedance (i.e., resistance) presented tothe driver circuit 724 may fall between 1Ω and 40Ω. Additionally, in anoperating mode, the imaginary load impedance (i.e., reactance) may bebetween 5j Ω and 48.7j (for example in the absence of multiplereceivers). In another embodiment, impedances presented to the drivercircuit 724 in an operating range may be from 4Ω to 40Ω and between −4jΩand 50jΩ Due to, for example, a varying number of wireless powerreceivers or other factors, the driver circuit 724 may be presentedloads with resistances in the 0 to 80Ω range and reactances from the−165jΩ to 95jΩ. It is desirable for the driver circuit 724 to operateefficiently and provide sufficient power to any load falling within thisrange. It is desirable to provide efficient and substantially constantpower over all these ranges given various design considerations.

In one aspect, a range of impedance values presented to the drivercircuit 724 may be defined by complex impedance values including realimpedance values and imaginary impedance values. The real impedancevalues may be defined or characterized by a ratio between a first realimpedance values to a second real impedance value. The ratio could beone of 2 to 1, 5 to 1, and 10 to 1. For example, the range of realimpedance values presented to the driver circuit 724 could be between 8Ωand 80Ω (a ratio of 10:1). In another embodiment, the range could bebetween 4Ω and 40Ω (also a ratio of 10:1). In another embodiment, therange could be between substantially 1Ω and substantially 200Ω. Inaddition, the range of impedance values presented to the driver circuit724 may be further defined by a range of imaginary impedance values. Therange of the imaginary impedance values may be defined as a ratio of themagnitude of the imaginary impedance values (i.e., magnitude betweenminimum and maximum imaginary impedance values) to a magnitude of thereal impedance values. For example, a magnitude of the real impedancevalues could be the magnitude of the difference between a first realimpedance value and a second real impedance value. The ratio of themagnitude of the imaginary impedance values to the magnitude of the realimpedance values may be at least one of 1:2, 2:1, 1:1, 2:3 etc. Forexample, if a real impedance range is between 8Ω and 80Ω, a magnitudemay be 72Ω. As such, if the ratio of the magnitude of the imaginaryimpedance values to the magnitude of the real impedance values is 2 to1, then the range of imaginary impedance values may be 144 (i.e., arange from −4jΩ to +140 jΩ). In any event, it is desirable to provideefficient and safe operation over a range of complex impedance valuesthat may be defined according to various methods.

As described above, the power and efficiency of a driver circuit 724 area function of the load the driver circuit 724 is driving. FIG. 8B is aplot showing efficiency 802 and output power 804 of the driver circuit724 of FIG. 7 as a function of the real impedance of a load (i.e., loadresistance) of the driver circuit 724. As shown in FIG. 8, 100% (ormaximum) efficiency at a single real load impedance value may exist(e.g., 50Ω as shown in FIG. 8) for an ideal class E amplifier. Theefficiency 802 decreases as the load impedance varies in eitherdirection. FIG. 8 also shows that the total output power 804 issimilarly a function of the load impedance and which peaks at particularload impedance value (e.g., 20Ω). Similar results are described in Raab,“Effects of Circuit Variations on the class E Tuned Power Amplifier”(IEEE Journal of Solid State Circuits, Vol. SC-13, No. 2, 1978).

If the driver circuit 724 drives a load with a constant impedance, thenthe driver circuit 724 may be ideally designed (e.g., values of thecapacitor 710 and inductor 708, etc. may be chosen) such that the drivercircuit 724 operates at maximum efficiency. For example, by using thevalues in the plot in FIG. 8B, if the driver circuit 724 is configuredto drive a load with an unvarying impedance that is substantially equalto 50Ω, the driver circuit 724 may drive the load at a maximumefficiency level. However, if the load of the driver circuit 724 varies,then the average efficiency and power delivered by the driver circuit724 may be significantly lower than its maximum efficiency or maximumpower as shown in FIG. 8. Furthermore, as the impedance of the loadincreases, the power delivered may not increase.

As shown in FIG. 7 and as described above, the load driven by the drivercircuit 724 may be a wireless power transmit circuit 750. The loadpresented to the transmit circuit 750, given a varying number ofwireless power receivers 608 a, 608 b, 608 c (FIG. 6), may thus vary theload seen by the driver circuit 724. In this case, the total loadimpedance presented to the transmit circuit 750 may be the sum of eachof the load impedances presented by each wireless power receiver 608 a,608 b, 608 c as they may combine in series. Ideally, the driver circuit724 would provide maximum efficiency over all loads while having thepower increase linearly as the resistance of the load increases. Powerwould then be divided among the loads. However, as seen in FIG. 8B,maximum efficiency for the driver circuit 724 may occur for a singlereal load impedance value.

One aspect of exemplary embodiments are directed to achieving highefficiency of the driver circuit 724 as the real load impedance varieswhile also increasing power as the load resistance increases. In oneaspect, this may allow for efficient wireless power transfer for avariable number of wireless power receivers 608 a, 608 b, and 608 c. Toprovide improved efficiency of a variety of loads, the efficiency of aclass E amplifier 724 is analyzed over variations in both a realcomponent of the load impedance (i.e., resistance) and the imaginarycomponent of the load (i.e., reactance). FIG. 9 is a contour plotshowing the efficiency of a driver circuit 724 as in FIG. 7 as afunction of the real and imaginary components of the load impedancepresented to the driver circuit 724. The plot may correspond to a drivercircuit 724 that is designed to have a maximum efficiency for a loadwith a resistance of 15Ω and a reactance of 0Ω In the illustratedembodiment, the drive voltage is 15 V. The complex load plot of FIG. 9shows efficiency contours 906 a, 906 b, and 906 c at increments of 5%.For example, points along the contour 906 a may represent thecombinations of resistance and reactance values that correspond to aload for which the class E amplifier is 95%. The contour 902 correspondsto load impedance values that correspond to an efficiency 100%.

The results of the plot shown in FIG. 8B may be seen in FIG. 9 byholding the reactance at zero and varying the resistance from 0 to 40Ωas shown by the arrow 908. The path 908 passes through the point 904with a value of 15Ω+j0Ω where efficiency is 100%. The contour 902 showsthat there is a path (e.g., a range of impedances) at which efficiencyis 100%. As such, rather than just analyzing efficiency over realimpedance values only, analyzing efficiency for both real and imaginaryimpedance values (i.e., a range of resistance and reactance values)shows that there is a range of complex impedance values for whichefficiency of the driver circuit 724 is 100%.

FIG. 10A is a contour plot showing the power output of a driver circuit724 as in FIG. 7 as a function of real and imaginary components of theload impedance presented to the driver circuit 724. The complex loadplot of FIG. 10A shows power contours 1006 a, 1006 b, and 1006 c at 1watt increments. For example, points along the contour 1006 b mayrepresent combinations of resistance values and reactance values thatrepresent an impedance value at which 5 watts of power may be delivered.Points along the contour 1006 c may represent combinations of resistancevalues and reactance values that represent an impedance value at which10 watts of power may be delivered. The results of the plot shown inFIG. 8B may be seen by holding the reactance at zero and varying theresistance from 0Ω to 40Ω as shown by the arrow 1008. The path 1008passes through the point 1004 where efficiency (shown by the contour 902from FIG. 9) is 100% and power delivered is a little over 6 Watts. The100% efficiency contour 902 of FIG. 9 placed in the plot of FIG. 10shows that there is path 902 where efficiency is 100% and where thepower continually increases as shown as the contours representincreasing power. As shown in FIGS. 9 and 10, the 100% efficiency path902 starts at an impedance of j24Ω, passes through 15+j0Ω and continuesto −j10Ω.

FIG. 10B is another plot showing power output and efficiency of a drivercircuit 724 as in FIG. 7 as a function of real and imaginary componentsof the load impedance presented to the driver circuit 724. The contour902 represents the combinations of resistance and reactance values forwhich efficiency is maximum, where efficiency may be defined as theratio of power delivered to the load divided by the DC power into theFET drain of the driver circuit 724. The contour 1006 represents theresistance and reactance values for which power delivered to the load isconstant for a particular drive voltage V_(d). While power may depend onboth the drive voltage V_(d) and the complex load, efficiency may dependon the load alone. The drive voltage V_(d) and power output may bechosen such that the power contour 1006 passes through the efficiencycontour 902 at the peak value of the load to the FET of the drivercircuit 724. Accordingly, the contours 1006 and 902 are exemplary andindicate that there is a range of complex values for which the drivercircuit 724 is efficient and for which power is constant. It is notedthat the contours shown in FIG. 10B may reflect the load seen by the FETof the driver circuit 724, Zload(FET) which may be offset from resultsthat would be in terms of Zin(TX). The difference may be due to a seriesinductance. However, Zload(FET) and Zload may be used interchangeably.FIG. 10B shows one particular results for when a drive voltage is on theorder of 10 volts and resulting power is on the order of 2.45 Watts. Inthis case values were selected to result in the power contour 1006passing through the efficiency circle at the peak value of Rload(FET) asshown. As a result when the Zload is 16.55+24.3j ohms, at 10 V dc, asingle sided driver circuit 724 delivers 2.45 watts with maximumefficiency into the real part of the load. These values are merelyexemplary and for purposes of illustration of values that may be foundto define the maximum efficiency and constant power contours.

As indicated above, however, a wide range of reactive and resistiveimpedances may be presented to the driver circuit 724 from a transmitcircuit 750 due to, for example, a variable number of receivers beingpositioned to receive power from the transmit circuit 750. Impedancevalues presented to the driver circuit 724 as a result of the variationof impedance presented to the transmit circuit 750 may result in reducedefficiency and fluctuations in the amount of power delivered.

FIGS. 11A, 11B, 12A, and 12B show corresponding measured results ascompared to FIG. 10 showing power output and efficiency of a drivercircuit 724 as in FIG. 7 as a function of real and imaginary componentsof the load impedance presented to the driver circuit 724. The measuredresults show power and efficiency contours as described above and whentaking into account various losses or other effects of the system. Forexample, the results may illustrate the effects of losses of the FET ofthe driver circuit (e.g., as compared to an ideal switch) and the effectof the resonant series LC circuit of the transmit circuit 750 betweenthe switch and the load. As such FIGS. 11A, 11B, 12A and 12B showmeasured efficiency and power contours when all load values within therange defined by R_(IN) _(—) _(TX) _(—) _(MIN)=0Ω, R_(IN) _(—) _(TX)_(—) _(MAX)=75Ω, X_(IN) _(—) _(TX) _(—) _(MIN)=−50 jΩ, and X_(IN) _(—)_(TX) _(—) _(MAX)=+50 jΩ are presented to a driver circuit 724. Theresults may reflect the output from a double-sided driver circuit 724(i.e., including two driver circuits 724 of FIG. 7) operating aparticular driver voltage (e.g., both driven by a 5.5 V DC supply). Inusing the double-sided driver circuit 724, load to each FET of thedriver circuit 724 is ½ that seen in FIGS. 11A, 11B, 12A, and 12B.

In accordance, FIG. 11A shows measured efficiency contours for a tuneddouble-sided driver circuit 724 without an added series inductance 708for the load range as noted above. FIG. 11B shows measured efficiencycontours for a tuned double-sided driver circuit 724 with an addedseries inductance 708. In accordance FIG. 12A shows measured powercontours for a tuned double-sided driver circuit 724 without an addedseries inductance 708 for a wide load range as described above. FIG. 11Bshows measured power contours for a tuned double-sided driver circuit724 with an added series inductance 708. Thus, FIGS. 11A-B showndifferent powers for the same load, illustrating the effect of V_(D) onpower. FIGS. 12A-B show output power.

It is noted that the measured efficiency and power contours of FIGS.11A, 11B, 12A, and 12B are merely exemplary for a particularconfiguration of driver circuit 724 and are used for purposes ofillustration. More specifically, FIGS. 11A, 11B, 12A, and 12B providevalues illustrating the range of complex impedance values for which thedriver circuit 724 is efficient and for which power is constant. It isnoted that the contour plots of FIGS. 11A, 11B, 12A, and 12B maycorrespond to results from a double-sided driver circuit 724configuration. To compare with a single sided driver circuit 724, thevalues would be divided by two. As compared to FIG. 10B, it is notedthat the constant power contours shown in FIGS. 12A and 12B are stillcircular in form, although distorted due to the effect of body diodesinherent in FET 704. Likewise, the efficiency contours shown in FIG. 11Aare similar to FIG. 10B, however efficiency values decrease as the loadresistance to the FET decreases which may be due to losses in the FET704.

Based on the results of FIGS. 9-12, certain aspects of exemplaryembodiments are directed to an impedance transform circuit (alsoreferred to herein as a filter circuit) located between the drivercircuit 724 and the transmit circuit 750 that transforms a variable loadimpedance presented to the transmit circuit 750 into values for whichthe driver circuit 724 is highly efficient and where power issubstantially constant. These values may be defined by the highefficiency and constant power contours as shown in FIGS. 9-12. Thevariable load presented to the transmit circuit 750 may vary widely overboth reactance and resistance as further described above. As will beindicated below, the transform circuit is configured to transform theimpedance to maintain the efficiency at a high level while keeping thepower transfer as constant as possible over a wide range in reactiveload. This may allow for a driver circuit 724 in a wireless powertransmitter 604 to efficiently provide power as the load presented tothe transmit circuit 750 varies reactively and resistively due to adynamic number of wireless power receivers 608 a, 608 b, and 608 c (FIG.6).

In one embodiment, a filter circuit is used to transform a variable loadimpedance presented to a transmit circuit 750 into complex load valuesfor which the driver circuit 724 may be highly efficient and for whichpower is constant. FIG. 13 is a schematic diagram of a driver circuit1324 as in FIG. 7 including a filter circuit 1326, in accordance withexemplary embodiments of the invention. The filter circuit 1326 (i.e., atransform circuit) is positioned between the driver circuit 1324 and thetransmit circuit 1350. As shown the transmit circuit 1350 shows avariable load 1312 including both variations in resistance andvariations in reactance. The filer circuit 1326 includes three reactivecomponents, X1 1328, X2 1330, and X3 1332. In some embodiments, X1 1328and X3 1332 are series inductors, while X2 1330 is a shunt capacitor.The filter circuit 1326 is configured to transform the impedance Zin(TX)presented by the transmit circuit 1350 into an impedance Zload(XFRM)presented to the driver circuit 1324 including the series inductor 1308.This impedance, Zload(XFRM) is then shifted by the series inductance1308 so as to “best” fit the load lines for which the driver circuit1324 is maximally efficient with a constant or steady power as the loadvaries. The full transformed impedance presented to the FET 1304 isdefined by Zload(FET). The component values for the filter circuit 1326and series inductance 1308 are configured to increase power linearlywith the resistive portion of the load presented the transmit circuit1350 and have as high efficiency as possible as the reactance varies.The values of the components are selected to provide a balance betweenmaximizing efficiency versus variation in the resistive portion Rin(TX)and minimizing load power variation versus variation in the reactiveportion Xin(TX).

While also operating as a low pass filter configured to reduce harmonicsin the signal, the filter circuit 1326 is configured to convert linearvariations in Zin(TX) to circular variations in Zin(XFRM). As notedabove, the series reactance 1308 shifts the load into a particular rangefor the FET 1304. The filter circuit 1326 is configured as a T network.While other configurations are also contemplated according to theembodiments described herein, the T network may reduce the number ofcomponents.

It is noted that according to some embodiments, where series inductorsare used for reactance components X1 1328 and X3 1332, high powerrequirements may increase the cost of the inductors. As such, it may bedesirable to eliminate one or more of the reactance components. Usingthe T network may allow for eliminating the reactance components X1 1328and X3 1332 which may be absorbed into the coil 1314 and/or the seriesinductor 1308. FIG. 14 is a schematic diagram of the circuit of FIG. 13in accordance with an embodiment. As shown, reactance component X2 1330of FIG. 13 is shown as a shunt capacitor 1430. Reactance components X1and X3 of FIG. 14 are absorbed into the coil 1414 and series inductor1408. As such, the filter circuit 1426 includes only of a shuntcapacitor. It is noted that the values of X1 and X3 are still selectedbased on the principles described herein to achieve the desiredimpedance transformation, and as such, the amount of the determinedvalues of X1 and X3 is taken into account when being absorbed into theother series elements. It is noted that the shunt capacitor 1430 couldbe replaced by a single shunt capacitor network (e.g., someconfiguration of parallel or series connected capacitors). However, onlya shunt network is needed. This may reduce the number of components andreduce other high cost inductors. The values of the reactive components1410, 1408, 1430, 1416, and 1414 are selected such that the circuit 1406is configured to transform the impedance into values that correspond tohigh efficiency for the driver circuit 1424 with constant power. Forexample, the shunt capacitor 1430 is selected based on an impedancetransformation ratio which is proportional to output power or current ata fixed supply voltage. The series L value 1408 may then be selectedfrom the shunt C value. As such the circuit 1406 may work efficientlyacross a wide reactance range while maintaining its output power, andreactance load switches to tune out the reactance swing may beunnecessary. As such, a single shunt capacitor network positionedbetween a class E amplifier and a resonant transmit circuit, whencombined with tuning adjustments of the existing inductances, canprovide a particular impedance transform as described herein. Thisincludes delivering power linearly versus the real load over a widereactance range, having reduced sensitivity to the reactive load changesand providing high efficiency for substantially maximum power.Furthermore, the value of the shunt capacitor 1430 may be selected todetermine an impedance ratio of the filter transform that may allowtrading the impedance ratio off against other factors such as mutualcoupling between a source coil (i.e., transmitter) and the load coil(i.e., receiver).

With reference again to FIG. 13, values of the components of the filtercircuit 1326 may be chosen such that the varying impedance of thetransmit circuit 1350 (due to receivers 608 a, 608 b, and 608 c) istransformed by the filter circuit 1326. The transformed impedance valuesmay correspond to impedance values (such as those as shown in FIGS. 9and 10) that provide highly efficient driver circuit 1324 operationwhile also increasing the flatness of power delivery given widereactance swings. The component values of the filter circuit 1326 arechosen to perform an impedance transform that transforms the impedanceof the load 1312 seen by the transmit coil 1350 into a complex impedancethat fits as closely as possible to complex values that provide highefficiency and constant power as shown in FIGS. 9-12. In someembodiments as will be further described below, the selection of theseries inductance 1308 of the driver circuit 1324 is used in conjunctionwith the filter circuit 1326 to shift the impedance transformationperformed by the filter circuit 1326 to match as closely as possible tocomplex values that provide high efficiency and constant power.

In one exemplary embodiment, the filter circuit 1326 may be configuredto modify the impedance presented to the filter circuit 1326 (e.g., theimpedance of the transmit circuit 1350 due to a variable number ofreceivers 608 a, 608 b, and 608 c) to maintain the efficiency of adriver circuit 1324 at a level that is within 20% of a maximumefficiency of the driver circuit 1324 within some complex impedancerange with real and reactive variations. In another embodiment,efficiency may be maintained at a level that is within 10% or lower of amaximum efficiency of the driver circuit 1324. The filter circuit 1326is further configured to maintain a substantially constant power outputlevel irrespective of the reactive variations within the compleximpedance range presented from the transmit circuit 1350. Moreover, thefilter circuit 1326 is configured to maintain a substantially linearrelationship between the power output level and the resistive variationswithin the complex impedance range. The filter circuit 1326 may bereferred to as or be configured as an impedance transformation network.The range of impedance values presented to the filter circuit 1326 thatare transformed by the filter circuit 1326 may be characterized by arange of complex impedance values. The range of complex impedance valuesmay be within a range defined by a first real impedance value and asecond real impedance value, where a ratio between the first realimpedance value to the second real impedance value is at least two toone. In addition, the range of reactive variation of the impedance maybe related to the real impedance range. For example, the magnitude ofthe range of reactive variation for which the filter is configured tomaintain efficiency at 20% of a maximum efficiency may be substantiallytwice the magnitude of the real impedance range. The center of thereactive impedance range may be substantially centered at theinstantaneous real impedance value (i.e., resistive) presented to thetransmit circuit. The filter circuit 1326 is further configured tomaintain a substantially constant power level irrespective of thereactive variations within the impedance range. In addition, the filtercircuit 1326 is configured to maintain a substantially linearrelationship between the power level and the real variations within theimpedance range. For example, the range of real (i.e., resistive)impedance values may be substantially between 8Ω and 80Ω or 4Ω and 40Ωhaving a ratio of 10 to 1. In this case, the range of imaginary (i.e.,reactive) may have a magnitude on the order of 144 and could spananywhere from −74 jΩ to +152 jΩ depending on an instantaneous value ofthe real impedance. In another embodiment, the range of real impedancevalues may be between substantially 1Ω and substantially 200Ω.Furthermore, in an embodiment, the range of imaginary impedances may bebetween substantially −200 jΩ and +200 j jΩ. Within this range of realand imaginary impedances (i.e., resistive and reactive), the filtercircuit 1326 and selection of series inductance 1308 is configured tomaintain the efficiency of the driver circuit 1324 within 20% of amaximum efficiency of the driver circuit 1324, maintain a substantiallyconstant power level irrespective of the reactive variations, and/ormaintain a substantially linear relationship between the power level andthe real variations within the impedance range.

The imaginary impedance range may also be defined by a first imaginaryimpedance value and a second imaginary impedance value. The firstimaginary impedance value and the second imaginary impedance value maydefine approximate minimum and maximum imaginary impedance values. Therange of imaginary impedance values (i.e., the magnitude of thedifference between the first imaginary impedance value and the secondimaginary impedance value) may be defined by a ratio of the magnitude ofthe imaginary impedance value to the magnitude of the real impedancevalue (e.g., equal to a magnitude of the difference between the firstreal impedance value and the second real impedance value). The ratio maybe at least one of 1:2, 2:1, 1:1, 2:3, 3:2, etc. For example, if themagnitude of the real impedance values is 72Ω, and the ratio is 2:1, themagnitude of the range of imaginary impedance values may be 144 jΩ(e.g., a range of a minimum to a maximum). In another example, in oneembodiment, the first real impedance value may be substantially 4Ω, thesecond real impedance value may be substantially 40Ω, the firstimaginary impedance value may be substantially −4 jΩ, and the secondimaginary impedance value may be substantially j50Ω. A wide range ofcomplex impedance values may be presented to the filter circuit 1126given the design parameters and the potential number of receivers. Assuch, ranges and ratios contemplated by various exemplary embodimentsdescribed herein may substantially vary from the specific examplesprovided herein.

According to certain embodiments, a passive or fixed filter circuit 1326(i.e., substantially all of the components of the filter circuit 1326may be passive circuit elements) as shown in FIG. 13 may be provided.The circuitry would have an absence of dynamic switching in of reactiveelements. As such, the filter circuit 1125 may not require controlsignals or other dynamic logic to control or configure the circuit asthe load changes during operation. This may reduce cost and complexityand may provide other benefits as will be appreciated by one/thoseskilled in the art.

In some embodiments, a driver circuit 1324 may generate harmonics of6.78 MHz, when the operating frequency of the driver circuits 1324 issubstantially 6.78 MHz. For various reasons, including for meetingregulatory requirements, the filter circuit 1326 may be furtherconfigured to reduce unwanted harmonics produced by the driver circuit1324. By using information derived from the plots such as FIGS. 9 and10, the filter circuit 1326 may be designed (in various embodiments) tomeet spectral emission mask requirements (via reducing harmonics),ensure that the load impedance at which maximum power may be deliveredis above the load at which maximum efficiency is achieved, and/or expandthe range of load impedance values for which the driver circuit 1324 ishighly efficient.

Exemplary Filter Circuit Operation

FIG. 15 is a plot showing the impedance transformed by the filtercircuit 1326 versus the impedance presented to the transmit circuit 1350as mapped to a high efficiency contour. For example, to illustrate theoperation of the filter circuit 1326, FIG. 15 shows an exemplary resultof a filter circuit 1326 configured to match the theoretical singlesided driver circuit 724 and having the load contours shown in FIG. 10B.While specific values are described, it is noted that these values aremerely exemplary and for purposes of illustration of the operation ofthe filter circuit for one particular driver circuit configuration, anda wide variety of other values are contemplated according to theprinciples described herein. Two circles 1502 and 1504 are shown in FIG.15. One circle 1502, centered at Rin(XFRM)=0+0 j, defines Zin(XFRM) foran Rin(TX) variation from 0 to 1000 ohms, in 5 ohm steps, withXin(TX)=0. The path proceeds in a counter clockwise direction as Rin(TX)increases. The radius of this circle (defined as Rd) is 16.55 ohms,selected to match the 100% efficiency theoretical contour 902 of FIG.10B. Since the markers are for Rin(TX) 5 ohms apart, it is noted thatZin(XFRM) peaks when Zin(TX)=25+j0 Ohms. This defines the impedancetransform ratio of the circuit, and can be set as desired. Forvariations in Rin(TX) given other values of Xin(TX), it is noted thatthe ratios result in circles that all pass through Zin(XFRM)=0, −16.55j.

The second circle 1505, which is orthogonal to the first circle 1502,defines Zin(XFRM) for an Xin(TX) variation from −1000 to 1000 in 5 ohmsteps, when Rin(TX) is held constant at 25 Ohms. It is centered atZin(XFRM)=16.55−16.55j Ohms which forces these two curves to intersectat Zin(XFRM)=16.55+0j ohms. For variations in Xin(TX) given other valuesof Rin(TX), it is noted that these result in circles that pass throughZin(XFRM)=0, −16.55j. This result is shown in FIG. 16, where Rin(TX) isvaried over a wider range than specified.

The impedance seen by the FET 1308 may be the same plot of FIG. 16shifted to the right by the added series inductance 1308. The resultsshown in FIG. 16 further shows that the curves that make up Zin of thetransform are independent of the transform selected. Only the mapping ofZin(XFRM) to Zin(TX) changes.

Given the observation from FIG. 12, to select the value of the seriesinductance 1308, it is decided where to locate the point where all thecurves, as seen by the FET 1304, meet. For example, FIG. 10B showsimpedances at FET 1304 to achieve 100% efficiency. In this case, Rd was16.55 Ohms, and the theoretical intercept point is at Zin(FET)=0+7.74jOhms.

FIG. 17 shows the path for an Rd equal to twice 16.55 Ohms, which wouldreduce the shunt capacitor 1310 on the FET 1304 also by a factor of 2.In this case the intercept is at Zload(FET)=0+15.5j Ohms.

To improve the match to the FET 1304 and thus achieve maximum efficiencyas Rin(TX) varies (but only for Xin(TX)=0), a series reactance 1308 isadded between the filter circuit 1602 and the FET 1304. FIG. 18 shows afinal result, superimposed on the theoretical power and efficiencycontours of FIG. 10B.

In an ideal result, the efficiency is 100% for all Rin(TX) whenXin(TX)=0, but the constant power contour is not a perfect fit. ForVd=10 Vdc, the power is predicted to be at a constant amount along thedotted path, and where the curves cross, but will increase for non-zeroXin(TX), since higher power regions are ever smaller circles locatedinside one another.

As such, FIGS. 15-18 illustrate the operation of the impedancetransformation by the filter circuit 1326 having components withconfigured values in accordance with exemplary embodiments.

Method of Selecting Filter Circuit Components

As indicated, the components of the filter circuit 1326 are selected toperform the desired impedance transform as described above. Inaccordance with an embodiment, a method is provided for determiningvalues of the components of the filter circuit 1326 that provides thedescribed impedance transform. In one aspect, the method is derived fromthe relationship that defines the transform from Zin(TX) to Zin(XFRM).Zin(TX) may be defined as R+jX and Zin(XFRM) as Zin(R,X). In this formZin(R,X) is:

${{Zin}\left( {R,X} \right)}:={{j \cdot X_{1}} + \frac{1}{\frac{1}{j{\cdot X_{2}}} + \frac{1}{{j \cdot X_{3}} + \left( {R + {j \cdot X}} \right)}}}$

According to the method, two variables are used to determine the filtercircuit reactances, X1 1328, X2 1330, and X3 1332. The first, variableRd that best fits Zin(R,X) to a measured efficiency contour as R varieswhile X is fixed at 0 ohms. The half circle defining these values iscentered at Zin(R,X)=0+j*0 and reaches a maximum of Rd at someZin(TX)=Rin0+j*0. The variable R0, which is defined as the value of R,when X=0, that results in Zin(R0, 0)=Rd. This defines the impedancetransform ratio of the filter circuit 1326, which may be done at onepoint, since this transform may be non-linear.

The method includes determining the efficiency and power contours vs.complex load at the FET 1304. This may be using a direct measurement toaccount for losses as described above. For example, exemplary contoursare shown in FIGS. 11A, 11B, 12A, and 12B that could be used accordingto this method. Note the test results to according to the method includethe capacitor 1310 that shunts the FET 1304. The total capacitance atthis node may determine the maximum efficiency region.

The method further includes determining if increasing efficiency vs.real load or holding power constant vs. imaginary load is morepreferable according to the particular desired operation. This sets theplacement of the circles generated by the transform including X1 1328,X2 1330 and X3 1332. The series reactance 1308 needed for input shiftingmay be determined subsequently. The series reactance needed for output(load) shifting can be accomplished by detuning the TX coil 1314.

The method further includes selecting the half circle path radius, Rd asdescribed above, for Zin(R,0) that will best fit the measured efficiencycontours. As noted, this half circle is centered at Zin(R,X)=0+j*0 andreaches a maximum of Rd at some Zin(TX)=Rin0+j*0.

The method further includes determining the full circle path thatcrosses this maximum value Rd of the half circle path. This circle isorthogonal to the constant efficiency contour, and lies approximatelyalong constant power contours. For Zin=Rin0+j*Xin, where Rin0 is fixedand Xin is variable, the resulting Zin(Rin0,Xin) will lie along thiscircle.

The method further includes selecting the value for R0 of the impedancewhere Zload will peak and is equal to Rd+0j. From Rd and R0, the valuesfor X1 1328, X2 1330 and X3 1332 may be determined according to thefollowing equations.

X ₁(R ₀):=(2·R _(d) ·R ₀)^(0.5) −R _(d)

X ₂(R ₀):=−(2·R _(d) ·R ₀)^(0.5)

X ₃(R ₀):=(2·R _(d) ·R ₀)^(0.5) −R ₀

In accordance, given Rd and R0, Zin(R,X) equals:

${{Zin}\left( {R,X} \right)}:={{- j} \cdot R_{d} \cdot \left\lbrack \frac{R + {j \cdot \left( {R_{0} + X} \right)}}{R - {j \cdot \left( {R_{0} + X} \right)}} \right\rbrack}$

Example of Filter Circuit Configuration

FIGS. 19-21 show results for on exemplary filter circuit 1326configuration in accordance with an embodiment. It is noted whilespecific numerical values are provided, the values are to illustrate onexample of a selection of components values for a filter circuit 1326,and other values for the components may be used in accordance with theprinciples described herein. The values of the filter circuit 1326 areconfigured to provide substantially constant power over a wide range inthe reactive load, X. The values are indicated as 2 times the valuesused for each side of the driver circuit 724 for a double-sidedconfiguration. The values for Rd and R0 for this exemplary filter werefound to be 17.6 and 23.3 respectively.

The resulting exemplary values for each single sideband transform are:

-   -   X1=11.0 Ohms, or L1=259 nH    -   X2=−28.6 Ohms, or C2=820 pf    -   X3=5.4 Ohms, or L3=125 nH

For the FET 1308, the value for Cshunt 1310 is selected as 2*287 pf.This includes each FET capacitance, estimated at 80 pf. For the filtercircuit 1326 it was determined to maximize the fit to the constant powercurves, for example as in FIGS. 12A and 12B. This results in an offsetfrom the maximum efficiency path and this fit comes by adding a seriesinductance 1308. From FIGS. 11A and 11B, the choice for the circleradius is 35.2 ohms. The resulting FET load paths peaks at Rin(TX)=46.6ohms, as desired.

The resulting transformed impedance for a double sided filter circuit1326 over the specified Zin(TX) load range is shown in FIG. 19. Thetransformed impedance is then shifted to align with the desired FETload, by Lseries 1308 of 2*516 nH. The double sided filter circuit 1326includes of a 2*820 nH series inductance, an 820/2 pf shunt capacitorand a 2*25 nH series inductance. This final series inductor can berealized simply by off-tuning the TX coil 1314 by +10.6 ohms.

It is noted that “straight line” impedances corresponding to variationsin the real impedance for a particular reactance are transformed intoorthogonal circles. This will be true for any filter circuit 1326 asshown in FIG. 13. Furthermore, it is noted that Zin(XFRM) givenZin(TX)=Rin(TX)+0*j. For this result, Rin(TX) varies as 0, 5, . . . 75ohms. Ideally Zin(ZFRM) reaches a peak of 35.2 ohms, with X=0 whenRin(TX)=45 ohms. In addition, the peak value and location of Rin(XFRM)changes as Xin(TX) changes. This is why the transform is designed withXin(TX)=0. Furthermore, the transform with Zin(TX)=45 ohms +j Xin(TX).Xin(TX) varies as −50, −40 . . . 50 ohms. Other points of FIG. 19 showall possible points that can be outcomes given the specified range ofZin(TX).

In order to overlay the transform prediction on the data of FIGS. 11A,11B, 11C, and 11D, the transform of FIG. 19 may be shifted to match themeasured results from the load box. There are two reactive shiftsinvolved. The first shift is to add two series reactances between theFETs and their filter circuits (for double-sided configuration). Thisadded reactance (Xshift) is combined with X1 1328 of the filter circuit1326 so that the total series inductance on each side is about 820 nH.

The second shift is to remove the two 600 nH series inductances used totest the driver circuit without any transform. After these shifts, thetransformed paths can be overlaid on the power and efficiency contoursmeasured for the FETs alone.

FIG. 20 shows the paths of the filter circuit 1326 for this exampleoverlaid on the measured data of FIG. 12B. As before, the half circle isfor fixed X=0, and variable R, and the full circle is with fixedRin(TX)=45 ohms and variable X. As can be seen the power vs. X contouris expected to be rather constant, while the power vs. variable R keepsincreasing with R. It is noted that the power where the lines cross canbe seen to be about 1.3 Watts, delivered into a Rload(FET) of 35 ohmswith Vd=5.5 Volts. By increasing Vd to 10 Volts, this power shouldincrease to about 4.3 Watts.

FIG. 21 shows the paths of the filter circuit 1326 for this exampleoverlaid on the measured data of FIG. 11B. As before, the half circle isfor fixed X, and variable R, and the full circle is with fixed R andvariable X. As can be seen, the efficiency is not at the maximum forRload=46, Xload=0. The maximum occurs around Rload=30, Xload=−20, whichdoes lie on the path of increasing Rin(TX).

It is noted that in addition to fitting the power and efficiencycontours, the methods described herein may allow to set the impedanceratio of the transformer.

In addition to the systems and methods disclosed herein above, attachedis one appendix: Appendix A. Appendix A is in 20 (twenty) pagesdescribing various configurations of a filter circuit 1326 that may beused by one or more of the methods and systems disclosed herein. Whilethe appendix may illustrate filter circuits having particular componentvalues, it is noted that the values are provided for illustration of theoperation and performance of the impedance transform and that thecomponent values for other embodiments selected according to theprinciples described herein may vary widely.

Further design characteristics of the driver circuit 1324 that may beused may include the driver circuit characteristic impedance, inputvoltage, and series reactance. In accordance with some embodiments, avariety of characteristics of the filter circuit 1326 may be chosen toarrive at a desired impedance transformation that may be correlated withthe high efficiency curve 902 and power contour. Characteristics of thefilter circuit 1326 may include the number of desired poles, the type offilter circuit, or a number of stacked filter circuits. The filtercircuit 1326 may be a ladder network of reactive elements that may takea variety of forms. For example, the ladder network may comprisemultiple reactive stages (i.e., reactive circuits) each including acombination of reactive components. Any of single value or multiplevalues of the ladder network may be adjusted based on a desiredresponse. Some filter circuits may be less desirable such as a filtercircuit 1326 that creates a simple reactance shift regardless of thecharacteristic impedance chosen. The ladder network may alsoinclude—more than three reactive elements, in which case all theseelements may be varied using a common parameter. Using multiple elementscan greatly reduce harmonics. However, it some cases it may be desirableto reduce the number of components for the filter circuit, for example,as described above with reference to FIG. 14.

Furthermore, as described above, the prototype class, the type offilter, the cutoff frequency, and the characteristic impedance of thefilter circuit 1326 may also be configured to achieve a desiredimpedance response that can be used. The prototype class may indicatehow the component values are chosen based on the other parameters. Thetype of filter circuit 1326 can be a low pass, high pass, band pass,notch, or combination thereof. The cutoff frequency may be a 3 dBattenuation point, although the cutoff frequency may vary depending onthe prototype class. The characteristic impedance may be the target realimpedance of the filter circuit 1326 if this were being used in a singleimpedance circuit (e.g., a 50Ω RF circuit).

According to one embodiment, given a set of several of thesecharacteristics (e.g., selecting the driver circuit design and drivercircuit filter series reactance 1108), non-selected characteristics(e.g., a filter circuit 1326 design) may be derived that allows thesystem to perform an impedance transformation of a range of real andreactive load impedances that transforms the real and reactive loadimpedance to a value for which the driver circuit 1324 is highlyefficient and for which power is constant.

Another method may include applying the reverse transformation of theseries reactance and ladder network (i.e., filter circuit 1326) to thehigh efficiency curve 902 and constant power.

As indicated herein, the driver circuit 1324 may behave as an ideal ACcurrent source (with a source impedance above some ratio of the maximumreal load) that supplies a constant current for a range of impedancesregardless of the impedance in that range presented to the drivercircuit 1324. The particular constant current may be chosen based on thecombinations of characteristics used. As such, the wireless powertransmitter 404 may be able to source more power as the resistance(i.e., real impedance) increases.

Once the component values of the filter circuit 1326 has beendetermined, if the elements of the filter circuits are altered withdifferent impedance values, the impedance transformation might notresult in the desired impedances for which the driver circuit 1326 isefficient and for achieving constant power. For example, an“undesirable” transformation may result from using low tolerancecomponents, for example, components which could vary 20 percent. In thisfilter design, while using similar components, a change in impedancevalue can fail to result in impedance transformation that allows for thedriver circuit 1324 to be efficient when a variety of complex impedancesare presented to the driver circuit 1324 by the transmit circuit 1350.

According to the configuration of the filter circuit, the values of theimpedances of the various elements are particularly chosen in such amanner to achieve the desired transformation as described above.Altering these impedance values for any given element, even by 5%,results in a significantly different impedance transformation. As such,the given impedance transformation as described above that results inmaintaining the driver circuit 1124 at a high efficiency (e.g., within20% of the maximum efficiency) is achieved after careful selection ofimpedance values for the filter elements according to the principlesdescribed herein.

FIG. 22 is a flowchart of an exemplary method for designing a highlyefficient transmit circuit. The transmit circuitry may be configured forwirelessly outputting power to charge or power a receiver device. Inblock 2202, a driver circuit 1324 may be selected that is configured tooperate at an efficiency threshold over a first range of compleximpedance values presented by a load to the driver circuit 1324. Basedon the characteristics chosen, in block 2204, a filter circuit 1326 maybe selected that is configured to perform an impedance transformation totransform an impedance presented to the filter circuit 1326 to a secondrange of complex impedance values that is correlated to the first rangeof complex impedance values. In block 2206, an impedance adjustmentelement 1308 may be selected that is configured to shift the secondrange of complex impedance values such that the impedance presented tothe driver circuit 1324 is substantially equivalent to impedance valuesof the first range of complex impedance values.

It should be further appreciated that the filter circuit 1326 may beconfigured to transform impedance values for other types of loads otherthan a transmit circuit 1350 and thus principles of various embodimentsmay be practice with a wide variety of loads. As such, embodimentsdescribed herein are not limited to providing wireless power, andexemplary embodiments in accordance with the invention may be applied inother situations where a driver circuit 1324 may drive a variable loadof any type having a range of impedance values. In some embodiments, thetransmit circuit 1350 may include a transmit coil (or loop antenna)configured to resonate at a frequency of the signal provided by thedriver circuit 1350. The transmit circuit 1350 may be configured towirelessly output power to charge or power a receiver 608 a, 608 b,and/or 608 c as described above. The transmit circuit 1350 may furtherbe configured to wirelessly transmit power to a plurality of receivers608 a, 608 b, and 608 c. Each of the receivers 608 a, 608 b, and 608 cmay alter the impedance seen by the transmit circuit 1350 such that thetransmit circuit 1350 may include a wide range of complex impedancevalues that may be transformed by the filter circuit 1326. The filtercircuit 1326 may transform the impedance value into a value that has anon-zero reactance such that it is a complex impedance value with a realportion corresponding to resistance and an imaginary portioncorresponding to a reactance.

In some embodiments, the filter circuit 1326 may be a passive circuitand may not require added logic or control signals to operate. Thefilter circuit 1326 may be a low pass filter circuit 1324. It should beappreciated that a wide variety of filter circuit configurations may beused in accordance with exemplary embodiments and may be selected asdescribed according to the principles herein.

The amount of power provided by the driver circuit 1324 may beconfigured to increase as an amount of the resistive portion of theimpedance seen by the driver circuit 1324 increases. This may allow forcontinually delivering higher power while maintaining efficiency as morewireless power receivers 608 a, 608 b, and 608 c receive power from thetransmit circuit 1350. Furthermore, the filter circuit 1326 may allowsuch that a magnitude of the impedance seen by driver circuit 1324 atwhich maximum power may be provided is higher than the magnitude of theimpedance seen by the driver circuit 1324 at which maximum efficiency ofthe driver circuit 1324 is provided. As such, the driver circuit 1324may perform as a constant current source over a range of resistances(i.e., real impedance values). As described above, the driver circuit1324 may be a class E amplifier or other amplifier such as switchingamplifier. The driver circuit 1324 may include other types of amplifiersas described above. Of note, however, in certain embodiments a class Dcircuit would act like a voltage source.

It should be further appreciated that while shown as a filter circuit1326, other types of circuits, components, or modules may be used toperform the type of impedance transformation as described above totransform a range of impedance values into a complex value for which adriver circuit 1324 is highly efficient, in accordance with theprinciples described herein.

As described with reference to FIG. 13, one of the functions of thefilter circuit 1326 is to remove unwanted harmonics produced by thedriver circuit 1324. In one aspect, the harmonics may result inundesired emissions from the transmit circuit 1350. As such, the filtercircuit 1326 may be configured to reduce emissions from the transmitcircuit 1350 to reduce emissions to meet spectral emission requirementsas well as perform the impedance transformation as described above. Forexample, as described above, the filter circuit 1326 may be a low passfilter that is seventh order capable to reject radiated emissions andconducted emissions of the transmit circuit 1350 and reduce couplingfrom a receiver to the transmitter. In one embodiment, the filtercircuit 1326 may be configured to reduce/reject radiated emissions andconducted emissions of the transmitter between substantially 20-250 MHz.It should be appreciated that the filter circuit 1326 may further beconfigured to reject emissions in other frequency ranges according todifferent applications and power requirements or different operatingfrequencies.

FIG. 23 is another schematic diagram of portion of transmit circuitry2300, in accordance with an exemplary embodiment. The transmit circuitry2300 shows an example comprises dual driver circuits 2324 in addition toother components for reduction of emissions. As shown, a first filtercircuit 2360 and a bypass capacitor 2370 configured to reduce emissionsfrom the driver circuit 2324 and the transmit circuit 2350 to the powersource 2302 are included. In some embodiments, the transmit circuitry2300 can include a shunt capacitor on V_(DD). The inductors at 2360 maybe a common mode choke. The driver circuit 2324 includes dual class Eamplifiers 2324 which drive the transmit circuit 2350 via a third filtercircuit 2323 as shown. The third filter circuit 2323 (dual filtercircuits) is configured perform the impedance transformation asdescribed above. The third filter circuit 2323 can be further configuredto reduce emissions of the transmit circuit 2350, as described above.

FIG. 24 shows measured results for power output a driver circuit 1324 asin FIG. 13 as a function of real and imaginary components of the loadimpedance presented to the driver circuit 1324. The measured resultsshow power contours as described above and when the impedanceZload(XFRM) is presented to the driver circuit 1324. As shown, the poweroutput is approximately independent of Xload, while increasing vs. Rloadat any Xload.

The various operations of methods described above may be performed byany suitable means capable of performing the operations, such as varioushardware and/or software component(s), circuits, and/or module(s).Generally, any operations illustrated in the Figures may be performed bycorresponding functional means capable of performing the operations. Forexample, means of transmitting may include a transmit circuit. Means fordriving may include a driver circuit. Means for filtering may comprise afilter circuit.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the exemplary embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the exemplary embodiments of the invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the exemplary embodiments disclosed herein may beimplemented or performed with a general purpose processor, a DigitalSignal Processor (DSP), an Application Specific Integrated Circuit(ASIC), a Field Programmable Gate Array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theexemplary embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. A software module may reside in Random AccessMemory (RAM), flash memory, Read Only Memory (ROM), ElectricallyProgrammable ROM (EPROM), Electrically Erasable Programmable ROM(EEPROM), registers, hard disk, a removable disk, a CD ROM, or any otherform of storage medium known in the art. An exemplary storage medium iscoupled to the processor such that the processor may read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor. The processor andthe storage medium may reside in an ASIC. The ASIC may reside in a userterminal. In the alternative, the processor and the storage medium mayreside as discrete components in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on a computerreadable medium. Computer readable media includes both computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. A storagemedia may be any available media that may be accessed by a computer. Byway of example, and not limitation, such computer readable media maycomprise RAM, ROM, EEPROM, CD ROM or other optical disk storage,magnetic disk storage or other magnetic storage devices, or any othermedium that may be used to carry or store desired program code in theform of instructions or data structures and that may be accessed by acomputer. Also, any connection is properly termed a computer readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer readable media.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The previous description of the disclosed exemplary embodiments isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these exemplary embodimentswill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the spirit or scope of the invention. Thus, the presentinvention is not intended to be limited to the exemplary embodimentsshown herein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

What is claimed is:
 1. A transmitter apparatus, comprising: a drivercircuit characterized by an efficiency and a power output level, thedriver circuit electrically connected to a transmit circuit having animpedance, the impedance of the transmit circuit within a compleximpedance range including resistive and reactive variations, the compleximpedance range defined by a minimum real impedance value, a maximumreal impedance, a minimum imaginary impedance value, and a maximumimaginary impedance value, a ratio between the minimum and maximum realimpedance value being at least two to one, a magnitude of the differencebetween the maximum and minimum imaginary impedance values being atleast twice a magnitude of the difference between the minimum andmaximum real impedance values; and a filter circuit electricallyconnected to the driver circuit and configured to modify the impedanceof the transmit circuit to maintain the efficiency of the driver circuitat a level that is within 20% of a maximum efficiency of the drivercircuit when the impedance is within the complex impedance range, thefilter circuit further configured to maintain a substantially constantpower output level irrespective of the reactive variations within thecomplex impedance range, and the filter circuit further configured tomaintain a substantially linear relationship between the power outputlevel and the resistive variations within the complex impedance range.2. The transmitter apparatus of claim 1, wherein the ratio is one of atleast five to one or at least ten to one.
 3. The transmitter apparatusof claim 1, wherein the minimum real impedance value is substantially 1ohm, the maximum real impedance value is substantially 50 ohms, theminimum imaginary impedance value is substantially −50 j ohms, and themaximum imaginary impedance value is substantially +50 j ohms.
 4. Thetransmitter apparatus of claim 1, wherein the minimum real impedancevalue is substantially 1 ohm, the maximum real impedance value issubstantially 100 ohms, the minimum imaginary impedance value issubstantially −100 j ohms, and the maximum imaginary impedance value issubstantially +100 j ohms.
 5. The transmitter apparatus of claim 1,wherein the transmit circuit comprises a transmit coil electricallyconnected to the output of the filter circuit, the transmit coilconfigured to wirelessly transmit power at a level sufficient to chargeor power one or more receiver devices.
 6. The transmitter apparatus ofclaim 1, wherein the transmit circuit is configured to wirelesslytransmit power at a level sufficient to charge or power one or morereceiver devices, and wherein positioning of the one or more receiverdevice to receive power from the transmit circuit causes the real andreactive variations in the impedance of the transmit circuit.
 7. Thetransmitter apparatus of claim 1, wherein the driver circuit comprises aclass E amplifier circuit comprising switch, a switch shunt capacitor,and a series inductor.
 8. The transmitter apparatus of claim 7, whereinthe filter circuit comprises one or more reactive components, whereinthe values of the one or more reactive components and the seriesinductor are selected to cause the modification of the impedance tomaintain the efficiency and the power output level.
 9. The transmitterapparatus of claim 8, wherein the one or more reactive componentsconsists of a single shunt capacitor network electrically coupled toground and between the driver circuit and the transmit circuit.
 10. Thetransmitter apparatus of claim 7, wherein the series inductor isconfigured to cause a reactive shift of the impedance between the drivercircuit and the filter circuit.
 11. The transmitter apparatus of claim1, wherein the minimum and maximum real impedance values correspond tothe resistive variations, and wherein the minimum and maximum imaginaryimpedance values correspond to reactive variations.
 12. The transmitterapparatus of claim 1, wherein the power output level increases as anamount of a resistive portion of the impedance increases.
 13. Thetransmitter apparatus of claim 1, further comprising: the transmitcircuit, comprising a coil having an inductance electrically connectedin series to a capacitor to form a resonant circuit, wherein the drivercircuit comprises a switching amplifier circuit comprising a switch, aswitch shunt capacitor, and a series inductor electrically connected tothe output of the driver circuit, the filter circuit electricallyconnected between the driver circuit and the transmit circuit, thefilter circuit comprising solely of a single shunt capacitor network.14. A transmitter apparatus, wherein the filter circuit comprises one ormore reactive components with values selected derived from: a firstvalue, Rd, corresponding to a radius of a half circle, the half circledefined by a set of complex impedance values along the perimeter of thehalf circle that correspond to values for which efficiency of the drivercircuit is at least within 20% of the maximum efficiency of the drivercircuit; and a second value, R0, corresponding to a real impedance valueat the load of the filter circuit that results in a desired transformedimpedance being equal to Rd at an input of the filter circuit.
 15. Amethod of selecting component values of one or more reactive componentsof a filter circuit for a wireless power transmitter device, the filtercircuit electrically connected between a driver circuit and a transmitcircuit, the method comprising: determining a first set of compleximpedance values for which efficiency of the driver circuit is above athreshold, the first set of complex impedance values substantiallymapping to complex impedance values along a half circle path;determining a second set of complex impedance values for which poweroutput of the driver circuit is substantially constant, the second setof complex impedance values substantially mapping to values along a fullcircle path that is orthogonal to the half circle and which crosses thehalf circle at a maximum; and selecting the component values to providean impedance transformation that modifies a variable complex impedanceof the transmit circuit to complex impedance values derived from thefirst and second sent of complex impedance values.
 16. The method ofclaim 15, wherein selecting the component values comprises: determininga first value, Rd, corresponding to radius of the half circle;determining a second value, R0, corresponding to a real impedance valueat a load of the filter circuit that results in a desired transformedimpedance between equal to the value Rd at the input of the filtercircuit; and selecting component values of the one or more reactivecomponents of the filter circuit based on the values derived from Rd andR0.
 17. The method of claim 16, wherein a first component value of afirst reactive component X1 is selected according to the equation:X1(R0)=(2·Rd·R0)^(0.5) −Rd.
 18. The method of claim 17, wherein a secondcomponent value of a second reactive component X2 is selected accordingto the equation:X2(R0)=−(2·Rd·R0)^(0.5).
 19. The method of claim 18, wherein a thirdcomponent value of a third reactive component X3 is selected accordingto the equation:X3(R0)=(2·Rd·R0)^(0.5) −R0.
 20. A method of adjusting an impedance of adriver circuit characterized by an efficiency and a power output level,the driver circuit electrically connected to a transmit circuit havingan impedance, the impedance of the transmit circuit within a compleximpedance range including resistive and reactive variations, the compleximpedance range defined by a minimum real impedance value, a maximumreal impedance, a minimum imaginary impedance value, and a maximumimaginary impedance value, a ratio between the minimum and maximum realimpedance value being at least two to one, a magnitude of the differencebetween the maximum and minimum imaginary impedance values being atleast twice a magnitude of the difference between the minimum andmaximum real impedance values, the method comprising: modifying theimpedance of the transmit circuit to maintain the efficiency of thedriver circuit at a level that is within 20% of a maximum efficiency ofthe driver circuit when the impedance is within the complex impedancerange; maintaining a substantially constant power output levelirrespective of the reactive variations within the complex impedancerange; and maintaining a substantially linear relationship between thepower output level and the resistive variations within the compleximpedance range.
 21. The method of claim 20, wherein the ratio is one ofat least five to one or at least ten to one.
 22. The method of claim 20,wherein the minimum real impedance value is substantially 1 ohm, themaximum real impedance value is substantially 50 ohms, the minimumimaginary impedance value is substantially −50 j ohms, and the maximumimaginary impedance value is substantially +50 j ohms.
 23. The method ofclaim 20, wherein the minimum real impedance value is substantially 1ohm, the maximum real impedance value is substantially 100 ohms, theminimum imaginary impedance value is substantially −100 j ohms, and themaximum imaginary impedance value is substantially +100 j ohms.
 24. Themethod of claim 20, further comprising wirelessly transmitting power ata level sufficient to charge or power one or more receiver devices. 25.The method of claim 20, further comprising wirelessly transmitting powerat a level sufficient to charge or power one or more receiver devices,and wherein positioning of the one or more receiver device to receivepower from the transmit circuit causes the real and reactive variationsin the impedance of the transmit circuit.
 26. The method of claim 20,wherein the driver circuit comprises a class E amplifier circuitcomprising switch, a switch shunt capacitor, and a series inductor. 27.The method of claim 26, wherein said modifying is at a filter circuitcomprising one or more reactive components, wherein the values of theone or more reactive components and the series inductor are selected tocause the modification of the impedance to maintain the efficiency andthe power output level.
 28. The method of claim 27, wherein the one ormore reactive components consists of a single shunt capacitor networkelectrically coupled to ground and between the driver circuit and thetransmit circuit.
 29. The method of claim 26, further comprising causinga reactive shift of the impedance between the driver circuit and thefilter circuit.
 30. The method of claim 20, wherein the minimum andmaximum real impedance values correspond to the resistive variations,and wherein the minimum and maximum imaginary impedance valuescorrespond to reactive variations.
 31. The method of claim 20, furthercomprising increasing a power output level as an amount of a resistiveportion of the impedance increases.